Day 9, oil, is better than day 9, cloth, as oil gives sizeable and substantial buffs to metal, a main resource from here on out. Oil starts with you drilling to reach oil, which starts slow. The heavy drill needs metal, like day 8's shears, and so I idled for a few minutes o get the metal, and started drilling. Initially I was concerned that this would be like day 8, how you needed lots of metal but it didn't give buffs to metal gain, as the first milestone for oil was a buff to coal gain, but the third milestone completely removed those worries, as we got 5% more metal per meter drilled, which will add up over time. the upgrades were also what you'd call "Nice" as they give major buffs to drill speed, which because of milestone 1 and 3, benefits both coal and metal, two of the main three resources we are concerned with in this day. the milestones also gain us "Heavy Drill Drill", which gives a logarithmically scaling buff to heavy drills, but this buff is strong, so it balances out. the 150m milestone gives a extra upgrade to coal and metal, which again, are two very important resources, and these upgrades benefit both coal and metal gain, which are very important as Heavy Drills require lots of coal to function. Funny enough I didn't have a problem in my game about coal consumption, despite a entire buyable being dedicated to reducing coal consumption at the cost of oil. at 350m we unlock the heavy Extractor, which trade drill speed for massive buffs to metal and coal, but these can be disabled so Heavy Extractor can help you at any stage of this day. we then unlock oil pump, which bases output on both drill speed and heavy extractors, basically, buying and day 8 drilling building buffs oil gain. the oil pump also increases coal consumption by a lot, but again, I never had this problem, this also brings into question why the developer added a buyable that is all about burning oil to reduce coal need. we then have oil upgrades, which are nice and strong, and we also have the Oil Smelter, which uses oil to better auto smelt the metal, which is something I never saw the benefit in. at 3000m we get the final milestone (for now) where it doubles Oil gain, this is nice and powerful. aside from that, it's pretty much just smooth expansion and sailing until you reach the end goal of the day. I would say that day 9 was pretty good, 6/10. (day 8 was 3/10).